In general, a dimming control system for a vehicular lamp includes: multiple light source units each having a semiconductor light source and a current control portion that controls a drive current for driving the semiconductor light source; and a control unit that is connected to the light source units via power supply lines. The control unit includes switch devices (switch portions) that control dimming of the light source units by repeatedly switching between ON and OFF in predetermined cycles.
Dimming of a vehicular lamp is controlled by detecting an external light and executing a control in accordance with the brightness of the detected external light. For example, brightness is required less in the evening than at night (at normal times). Therefore, the dimming control of the vehicular lamp is performed such that the brightness of the headlamp is slightly darker than normal.
Abnormality in the power supply lines or the light source units when the light source units are lit is determined by detecting a current value or a voltage value supplied via the power supply lines and determining whether the detected current value or voltage value exceeds a predetermined value. The determination is performed by an abnormality determination process program stored in a CPU (central processing unit) in the control unit.